Photovoltaic power generation is a known way of generating power by converting sunlight into power. In photovoltaic cell modules used in photovoltaic power generation, the generated power has a peak with respect to the current or voltage (see FIG. 7(a)). In order to increase power generation efficiency, there is a demand for power to be generated at the current or voltage at which the generated power peaks.
The peak of generated power varies according to a variety of factors such as sunlight conditions, temperature, and the like. Therefore, in order to improve the power generation efficiency, control for maximum power point tracking is performed by generating power while tracking the maximum power point, i.e. the current or voltage that maximizes the generated power.
Normally, a hill climbing method is used as control for maximum power point tracking. When the peak position of the current or voltage that causes the generated power to peak simply varies, the hill climbing method can be used to cause the photovoltaic cell module to generate power at the maximum power point.
In an abnormal state in which a shadow is cast on a portion of the photovoltaic cell module, however, there may be multiple peaks in the generated power (see FIG. 7(b)). When there are multiple peaks, it might not be possible to generate power at the actual maximum power point with the hill climbing method.
It is possible to distinguish the maximum among a plurality of peaks in the generated power with a maximum power point search, in which the maximum power point is searched for by measuring the actual generated power while varying the current or voltage over the entire adjustable range thereof. During a maximum power point search, however, power is generated even in a state when the generated power is small. Therefore, the power generation efficiency may be reduced due to the performance of an unnecessary maximum power point search.
An unnecessary maximum power point search can be prevented by performing the maximum power point search in an abnormal state. It has thus been proposed to distinguish an abnormal state of a photovoltaic cell periodically (see JP 2012-186409 A (PTL 1)).